1. Field of the Invention
The present invention relates to a silver halide photographic lightsensitive material. More specifically, the present invention relates to a highly sensitive, low-fogging silver halide photographic lightsensitive material.
2. Description of the Related Art
A silver halide photographic lightsensitive material mainly comprises a dispersion medium containing lightsensitive silver halide grains applied on a support. To increase the sensitivity of silver halide lightsensitive materials, an enormous amount of study has been made. In order to enhance the sensitivity of a silver halide lightsensitive material, it is very important to increase the sensitivity inherent to the silver halide grains. For increasing the sensitivity of silver halide grains, various methods are employed. Enhancement of sensitivity are accomplished, such as enhancement of sensitivity using chemical sensitizers such as sulfur, gold and compounds of the VIII Group; enhancement of sensitivity using a combination of chemical sensitizers such as sulfur, gold and compounds of the VIII Group, and additives that facilitate the sensitizing effect of the chemical sensitizers; and enhancement of sensitivity by the addition of an additive having an sensitizing effect depending on a kind of silver halide emulsion. Descriptions on these methods can be found in Research Disclosure, Vol. 120, April, 1974, 12008, Research Disclosure, Vol. 34, June, 1975, 13452, U.S. Pat. Nos. 2,642,361, 3,297,446, 3,772,031, 3,857,711, 3,901,714, 4,266,018 and 3,904,415, and British Patent No. 1,315,755. Further, a method comprising reduction-sensitizing silver halide grains is also employed as a method for enhancing sensitivity. Reduction-sensitization of silver halide grains is disclosed in, for example, U.S. Pat. Nos. 2,518,698, 3,201,254, 3,411,917, 3,779,777 and 3,930,867, and a method of using a reducing agent is disclosed in, for example, Jpn. Pat. Appln. KOKOKU Publication No. (hereinafter referred to as JP-B-) 57-33572, JP-B-58-1410, and Jpn. Pat. Appln. KOKAI Publication No. (hereinafter referred to as JP-A-) 57-179835. Furthermore, a sensitizing technique using an organic electron-donating compound comprising an electron-donating group and a leaving group has been reported as described recently in U.S. Pat. Nos. 5,747,235 and 5,747,236, EP Nos. 786692A1, 893731A1 and 893732A1, and WO99/05570. This is a novel sensitizing technique and is effective in enhancement of sensitivity. However, although the use of this compound results in an enhanced sensitivity, it has also the defect that the fogging or Dmin becomes high, and therefore improvement has been desired.